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Pull-down resistor for negated pin...

Discussion in 'Electronic Design' started by Franco, Oct 2, 2007.

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  1. Franco

    Franco Guest

    Hi to everyone:

    I was showing an electronic design to one friend and he told me I
    should always use a "pull-down" resistor for permanently grounding a
    negated pin in a chip (previously I had it connected to GND - 0V).

    Can someone explain the reason of that? or some source of information?

    Best Regards...

    Franco.
     
  2. Joel Kolstad

    Joel Kolstad Guest

    The idea is usually just that it's a lot easier to undo if you change your
    mind and need to use the pin for something after all. :)

    For engineering prototypes, it's one of those things where it "can't hurt,
    might help." For production boards, at least for high volumes, most people
    would choose to save the money by tying to ground as you've done.
     
  3. Franco

    Franco Guest

    Great and honest answer.

    This pin is normally driven by a uC, so if I want to change the design
    I will connect the uC which has an internal pull-up resistor in this
    case.

    I have a big problem with the space in this board (it is a 4 layer
    25mm diameter circle!!!). So, even an 0603 SMD component is painful.

    Best Regards...

    Franco.
     
  4. You may also be able to program the pin as an output and leave it
    floating. If it cannot be programmed as an output, then there is
    little reason to use a resistor.
    Some kind of RF or RFID gadget?

    Best regards,
    Spehro Pefhany
     
  5. Franco

    Franco Guest

    Mmmm, no. It is a little controller board for a robotic application.

    Thanks for the answers...

    Franco.
     
  6. Tam/WB2TT

    Tam/WB2TT Guest

    An automated test machine will want to drive any pin either high or low. It
    can't drive a pin that is connected to either gnd or vcc. 220 Ohms is a good
    value for a pulldown. Same goes for unused functions; you will want these to
    work.

    Tam
     
  7. JosephKK

    JosephKK Guest

    Tam/WB2TT [email protected]$t.net posted to sci.electronics.design:
    Not really. I do not know of any that were not "fully programmable"
    even 25+ years ago. They tested what they were told to. The
    expensive complex ones, even then, had a modest computer (high range
    pdp-11's running rsx-11) running the controls of the hardware and
    logging the test results. And "bed of nails" tester configurations
    were being abandoned in favor of early JTAG; the speeds were starting
    to really climb and pin driver electronics and fixturing was getting
    really difficult.
     
  8. Tam/WB2TT

    Tam/WB2TT Guest

    I agreee with you, but pin testing is where the rules concerning pullups and
    pulldowns came from.

    Tam
     
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